Generally, there is an example common working mechanism for construction machines, part of which is moved (offset), in parallel, to the left or right side of the vehicle body. Such a working mechanism is employed for offset boom type hydraulic excavators, etc. (e.g., Japanese Patent Laid-Open No. 2003-96810 and No. 2001-317076).
According to these prior arts, this type of working mechanism for a hydraulic excavator is constituted by three booms, i.e., first, second and third booms, and the first boom is mounted on the vehicle body of the hydraulic excavator as to be rotatable to the upper and lower sides. Further, the second boom is mounted on the distal end of the first boom as to be swingable to the left and right sides, and the third boom is mounted on the distal end of the second boom as to be swingable to the left and right sides. Furthermore, an arm is provided at the distal end of the third boom, and a working tool, such as a bucket, is attached at the distal end of the arm.
The working mechanism also includes a boom cylinder which moves the first boom to the upper and lower sides, an offset cylinder which swings the second boom to the left and right sides, and an arm cylinder and a bucket cylinder which rotate an arm and a bucket.
According to the prior arts, single link in the shape of an elongated rod is provided, for example, on the left side of the second boom to connect the first and the third booms, and the link and the second boom constitute a parallel linking mechanism.
When the second boom is moved to the left side, the third boom located at the distal end of the second boom is pushed to the right side by the link. Then, the third boom, the arm, and the bucket are moved (offset) to the left side of the vehicle body, while their postures parallel to the first boom are maintained. Further, according to the arrangement, when the second boom is moved to the right side, the third boom is pulled to the left link, and the third boom, the arm, and the bucket are moved horizontally to the right side of the vehicle body.
According to the above described prior arts, when the second boom is moved to the left or right side, the third boom is pushed or pulled by one link. Therefore, in accordance with the direction in which the second boom is moved, a great force in the compression direction or in the pulling direction is exerted on the link. Further, when an external force to the left or right side is exerted on the third boom, the arm, and the bucket, the force in the compression direction or the force in the pulling direction is exerted on the link via the third boom.
Therefore, according to the prior arts, one link must have sufficient strength relative to the forces that are exerted in two directions, in the compression direction and in the pulling direction (especially, the force exerted in the compression direction). Therefore, for example, a large cross-sectional area (the outer diameter) and a large weight for the link are required, and accordingly, there is a problem that the size of the entire working mechanism is increased.
In addition, when the outer diameter of the link is increased, the gap between the second boom and the link is narrowed. Thus, when the third boom is moved to the left or right side within a range wherein the second boom and the link do not interfere with each other, there is a problem that the traveling distance (an offset value) is limited.
Further, when a large gap is to be provided between the second boom and the link, the forces exerted on the link in the compression direction and in the pulling direction can be reduced, however, the size of the working mechanism is increased to the left and right sides in accordance with the distance between the link and the second boom.